As we are using a symbol map for libsandbox, its no longer needed
to have all the internal functions static, and thus we can break
things out a bit and make the source layout more sane. Start by
moving librcutil to libsbutil, and adding all the defines and
helper functions needed by both libsandbox and sandbox.

Rename sandbox_futils.c to sandbox_utils.c. Add gstrndup() and gbasename()
to sandbox_utils.c. Add check for glibc, and fixup things to not need glibc
only extensions if not needed for versioned symbols.

Fix inverse test logic in canonicalize.c, use a strncpy. Fix gcc warning in
getcwd.c. Add symbols.in and logic to Makefile.am to generate symbol versions
for glibc and other libc's that use this. Update libsandbox.c to use these
symbol versions if available. Fix exec wrapper to re-export LD_PRELOAD if the
process unset it.